1. Field of the Invention
The present invention relates to a refrigerating apparatus which is adapted to refrigerating machines and equipments such as air conditioners, refrigerators, freezers, show-cases equipped with a freezer or a refrigerator so as to control the degree of superheat and a method of controlling such a refrigerating apparatus in accordance with a fuzzy reasoning, i.e., fuzzy logic.
In conventional refrigerating apparatuses of this type, an expansion valve which is driven and the opening of which is controlled by a pulse motor is provided between a condenser and an evaporator in a refrigerant circuit, and the opening of the expansion valve is so controlled as to keep the degree of superheat constant which is obtained from the difference between the temperature at which the refrigerant is evaporated by the evaporator and the exit temperature of the evaporator, thereby preventing the liquid refrigerant from flowing back to the compressor, as described in, for example, U.S. Pat. Nos. 4,674,292 and 4,745,767.
In the controlling methods described in the above-described specifications, what is called a PID control system is adopted. The PID control system consists of P control for controlling the deviation of the measured degree of superheat from the preset degree of superheat so as to cancel it by an output which is proportional to the deviation, D control for controlling the deviation so as to cancel it on the basis of a change of the deviation, namely, the differential coefficient of the deviation, and I control for controlling the deviation so as to cancel it on the basis of the accumulation of the deviation, namely, the integrated value of the deviation.
Such a PID control system is defective in that PID control cannot follow well a transient change, especially, after the defrosting of the evaporator, which causes the liquid refrigerant to flow back to the compressor or prolongs the overheated state. To prevent this, a measure of making the proportional constant variable is taken. In this case, however, the control becomes so sensitive that the controlling stability is deteriorated.